It is important to detect a fuel injection condition, such as a fuel-injection-start timing, a maximum fuel injection rate, a fuel injection quantity and the like in order to accurately control an output torque and an emission of an internal combustion engine. Conventionally, it is known that an actual fuel injection condition is detected by sensing a fuel pressure in a fuel injection system, which is varied due to a fuel injection. For example, JP-2008-144749A (US-2008-0228374A1) describes that an actual fuel-injection-start timing is detected by detecting a timing at which the fuel pressure in the fuel injection system starts to be decreased due to a start of the fuel injection and an actual maximum fuel injection rate is detected by detecting a fuel pressure drop (maximum fuel pressure drop).
A fuel pressure sensor disposed in a common rail hardly detects a variation in the fuel pressure with high accuracy because the fuel pressure variation due to the fuel injection is attenuated in the common rail. JP-2008-144749A and JP-2000-265892A describe that a fuel pressure sensor is disposed in a fuel injector to detect the variation in the fuel pressure before the variation is attenuated in the common rail.
The present inventors has studied a method of computing the maximum fuel injection rate based on a pressure waveform detected by the pressure sensor disposed in a fuel injector, which method will be described hereinafter.
As shown in FIG. 15A, when a command signal for starting a fuel injection is outputted from an electronic control unit (ECU) at a fuel-injection-start command timing “ls”, a driving current pulse supplied from an electronic driver unit (EDU) to a fuel injector starts to rise at the fuel-injection-start command timing “ls”. When a command signal for ending a fuel injection is outputted from the ECU at a fuel-injection-end command timing “le”, the driving current pulse starts to fall at the fuel-injection-end command timing “le”. A detection pressure detected by the fuel pressure sensor varies as shown by a solid line “L1” in FIG. 15B.
It should be noted that the command signal for starting a fuel injection is referred to as a SFC-signal and the command signal for ending a fuel injection is referred to as an EFC-signal, hereinafter.
When the SFC-signal is outputted from the ECU at the fuel-injection-start command timing “ls” and a fuel injection rate (fuel injection quantity per unit time) increases, the detection pressure starts to decrease at a changing point “P3a” on the pressure waveform. Then, when the EFC-signal is outputted at the fuel-injection-end command timing “le” and the fuel injection rate starts to decrease, the detection pressure starts to increase at a changing point “P7a” on the pressure waveform. Then, when the fuel injection ends and the fuel injection rate becomes zero, the increase in the detection pressure ends at a changing point “P8a” on the pressure waveform.
Timings “t1” and “t3” at which the changing points “P3a” and “P7a” respectively appears are detected and the fuel pressure drop (maximum fuel pressure drop “Pβ”) generated during a period from a timing “t1” to a timing “t3” is computed. Since the maximum fuel pressure drop “Pβ” and the maximum fuel injection rate have a high correlation with each other, the maximum fuel injection rate is computed based on the maximum fuel pressure drop “Pβ”.
Specifically, as shown by a solid line M1 in FIG. 15C, differential values are computed with respect to every detection pressure. After the SFC-signal is outputted at the fuel-injection-start command timing “ls”, the differential value first becomes lower than a threshold TH at a timing “t1”. This timing “t1” is detected as the timing at which the changing point “P3a” appears. The timing “t1” corresponds to a fuel-injection-start timing. Further, after the fuel-injection-star timing “t1”, the differential value first becomes zero at a timing “t3”. This timing “t3” is detected as the timing at which the changing point “P7a” appears. This timing “t3” is referred to as a maximum-fuel-injection-rate-reach timing. The fuel pressure at timing “t3” is subtracted from the fuel pressure at timing “t1” to obtain the maximum fuel pressure drop “Pβ”.
It should be noted that the pressure waveform illustrated by the solid line L1 in FIG. 15B represents a waveform in a case that a single fuel injection is performed during one combustion cycle. In a case that a multi-stage injection is performed, the pressure waveform generated by the second or successive fuel injection is illustrated by a broken line L2. This pressure waveform illustrated by the broken fine L2 is generated by overlapping an aftermath (refer to an encircled portion “A0” in FIG. 15B) of the previous waveform with the current waveform. When such a pulsation is generated in the pressure waveform, the differential values shift from the solid line M1 to a dashed line M2 in FIG. 15C. That is, the differential value becomes lower than the threshold TH at a timing “tx”, and this timing “tx” which is earlier than the actual fuel-injection-start timing “t1” is erroneously detected as the fuel-injection-start timing. The fuel pressure at the changing point “P3a” is erroneously detected as the fuel pressure at the changing point “P3x”. The maximum fuel pressure drop “Pβ” can not be accurately computed.
Especially, in a case that a multi-stage injection is performed, when an interval between n-th injection and (n+1)th injection is short, an unstable pressure waveform of n-th fuel injection overlaps with the pressure waveform of (n+1)th fuel injection. The pulsations of the pressure waveform and the differential value become large and an erroneous detection may be caused.
Also, the fuel pressure at the changing point “P7a” is erroneously detected and the maximum fuel pressure drop “Pβ” can not be accurately computed.
Moreover, it is conceivable that noises overlapping on the pressure waveform may cause a disturbance of the pressure waveform. Thus, even in a case that single-stage injection is performed or the interval is long, the above mentioned erroneous detection may be performed.